(i) Field of the Invention
The present invention relates to lead-free ceramic glazes and, more particularly, to such glazes which can be applied to a ceramic article such as a spark plug insulator at lower application firing temperatures.
(ii) Description of the Related Art
Glazes are continuous coatings which are usually prepared from fused silicate mixtures and are fusion-bonded to ceramic substrates. A glaze is used on a ceramic substrate to serve one or more of the following purposes: (1) to render the substrate impermeable to liquids and gases; (2) for aesthetic reasons, including covering blemishes and providing decorative effects; (3) to provide protective coatings; and (4) to increase strength.
The exterior portion of a spark plug insulator is exposed to dirt and grease which may result in the formation of an electrically conducting surface and premature failure of the spark plug. Alumina insulator bodies of spark plugs are usually glazed in order to minimize dirt and grease build-up, and to increase the strength and imperviousness of the surface. Depending on the particular properties desired, the glaze can be modified to change the maturing temperature, to add color, or to modify the coefficient of thermal expansion.
Glazes applied to alumina substrates must have a low thermal coefficient of expansion, similar to that of the alumina substrate, to avoid undue stresses which can cause spalling, chipping, cracking or crazing of the glaze; from 6 to 7 microinches per inch per .degree.C. is a typical range of coefficient of thermal expansion for alumina bodies. A glaze with a low coefficient of thermal expansion also strengthens the insulator by inducing compressive stresses at the surface of the glaze-insulator composite. Because glazes involve highly complex multi-component systems, it is difficult to predict the effect of varying or substituting chemical compounds in a glaze formulation, even though general properties of some of the individual components are known. Furthermore, because a glaze is not homogeneous, that is, it may contain one or more dispersed undissolved phases, the ultimate components shown by chemical analysis do not describe a glaze such that the properties are easily predictable.
Because the oxides and carbonates of lead enter into combination with silica and boric acid, lead traditionally had found extensive use in glazes, although over the past decades such usage has been diminished in view of the known toxicity of lead. To this end, numerous lead-free glaze compositions have been developed.
For example, in U.S. Pat. No. 4,084,976, a lead-free glaze composition was disclosed comprising from 50 to 54% by weight SiO.sub.2, from 5 to 8% by weight Al.sub.2 O.sub.3, from 6 to 12% by weight B.sub.2 O.sub.3, from 4 to 6% by weight CaO, from 2 to 8% by weight MgO, from 2 to 15% by weight BaO, from 5 to 8% by weight SrO, from 1 to 2% by weight ZnO, and from 4 to 6% by weight Na.sub.2 O, K.sub.2 O, and Li.sub.2 O. The composition was applied to an alumina body and fired at 2120.degree. F. (1160.degree. C.) to produce a smooth, uniform coating of high gloss and good strength with no evidence of crazing or devitrification.
Another lead-free glaze composition is disclosed in U.S. Pat. No. 4,120,733. Such composition comprises 48 to 54% SiO.sub.2, from 7 to 11% Al.sub.2 O.sub.3, from 16.5 to 20% B.sub.2 O.sub.3, from 11 to 14% BaO, from 2 to 3% CaO, from 2 to 2.5% ZnO, from 4.25 to 5.25% Na.sub.2 O, and from 0.4 to 1% K.sub.2 O, Li.sub.2 O and MgO. Such composition differs from that described in U.S. Pat. No. 4,084,976 and, as a consequence of such differences, matures at temperatures from 80.degree. to 100.degree. F. (44.degree. to 56.degree. C.) lower, when fired under the same conditions. Such lower firing temperature was said to minimize deterioration of kilns and kiln furniture, and reduce the fuel requirements for the production of glazed, alumina bodies.
Finally, U.S. Pat. No. 4,256,497 relates to a lead-free glaze for alumina bodies including 35 to 54% SiO.sub.2, 7 to 11% Al.sub.2 O.sub.3, 17 to 25% B.sub.2 O.sub.3, 2 to 3% CaO, 2 to 2.5% ZnO, 4.25 to 5% Na.sub.2 O, 8.5 to 21% SrO, and 0.7 to 1% of a mixture of K.sub.2 O, Li.sub.2 O, and MgO. Unlike the previous two lead-free glazes described, that of U.S. Pat. No. 4,256,497 eliminates the BaO component and substantially increases the content of SrO. As a consequence of this difference in composition and corresponding alterations of the amounts of other components of the glaze composition, several specific advantages are realized including lowered toxicity of the SrO-containing glaze as compared with the lead-free glaze containing BaO, thereby further reducing the risk of occupational exposure, lower cost of SrO as compared with BaO, and greater availability of SrO and strontium-bearing minerals when compared to that of baria.
The above composition is coated onto an alumina ceramic and fired at temperatures ranging from 1800.degree. to 2200.degree. F. (982.degree. to 1204.degree. C.). A smooth, uniform glaze of high gloss and good strength was formed and there was observed no evidence of crazing or devitrification in the glaze.
The above discussion makes clear that two significant advancements have been made in the area of ceramic glazes over the past two decades. First, the art has successfully removed lead from such compositions. Second, the art has been able to achieve reductions in the temperatures at which such glazes must be fired after they are applied to a ceramic substrate.
Despite such advancements, however, still further reductions in firing temperatures for lead-free glazes are being sought in the art. More particularly, in the manufacture of a spark plug, it has been necessary to carry out two separate firing steps on an already sintered bisque alumina insulator. First, it has been necessary to glost fire raw-glazed insulators at 2050.degree.-2150.degree. F. (1121.degree.-1177.degree. C). By "raw-glazed insulators" is meant the ceramic component of the spark plug. It has then been necessary to carry out a second firing step at reduced temperatures, e.g., between 1550.degree. and 1650.degree. F. (843.degree. and 899.degree. C.), to incorporate a Carbon-based, Fired-In Suppressor glass seal that results in the production of spark plug core assemblies. By "core assembly" is meant a glazed insulator with a contained internal center electrode component. Reduced temperatures are required in view of the temperature-sensitive nature of certain of the components of the Carbon-Based Fired-In Suppressor glass seal. Quite clearly, the necessity for carrying out two separate firing steps adds to both the cost and the time involved in glazing and glass sealing the unit. It would therefore be advantageous to provide a lower-temperature lead-free glaze which could be co-fired in a single step with the Carbon-Based Fired-In Suppressor glass seal.